This invention relates to a method, apparatus, and system for fast base station synchronization and sector identification, particularly in a spread spectrum communication system.
With the growing trend towards globalization in communications, there has arisen a need for global communication standards. To meet this need, the International Mobile Telecommunications (IMT) 2000 standard is being developed. In the development of the IMT2000 standard, various techniques are under consideration for channel access, including Time Division Multiple Access (TDMA), Wideband Code Division Multiple Access (W-CDMA), and a hybrid of these techniques.
In a TDMA system, each channel is assigned a specific time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. In a CDMA system, different users, base stations (BS), and services are separated from each other with unique spreading sequences/codes.
In one kind of CDMA system, the informational datastream to be transmitted is impressed upon a much higher bit rate datastream generated by a pseudorandom code generator. The informational datastream and the high bit rate datastream are typically multiplied together. This combination of the higher bit rate signal with the lower bit rate datastream is called coding or spreading the informational datastream signal. The rate of the spreading code is referred to as the xe2x80x9cchip ratexe2x80x9d. The chip rate divided by the channel symbol rate is referred as the xe2x80x9cspreading factorxe2x80x9d (sf).
A plurality of coded information signals are combined and modulate a radio frequency carrier wave that is transmitted, and the plurality of signals are jointly received as a composite signal at a receiver. In a system in which several users are transmitting using different spreading codes, the resulting signal is a composite signal with the different coded signals overlapping both in time and frequency. By correlating the composite signal with one of the unique spreading codes, the corresponding information signal is isolated and decoded. This type of CDMA system is sometimes referred to as a Direct Sequence system.
In another kind of CDMA system, a technique referred to as M-ary Orthogonal Keying (MOK) data modulation is used. According to this technique, one basic spreading function, e.g., a pseudo noise (PN) sequence, is phase modulated on a carrier. The spreading function is modified by other functions, such as Walsh functions, to cause a modulation orthogonal to the basic function and every other modulating function used. The orthogonal keyed data is recovered, e.g., by using parallel decoding devices, such as correlators or convolvers.
In order to set up a connection between a remote station, e.g., a mobile station (MS) or a fixed terminal, and the cellular system, the remote station must identify and synchronize with at least one BS from which the remote station can receive signals and to which the remote station can transmit signals. Each BS may serve one cell with an omnidirectional antenna or with one or more directional antennas. Since the quality of a connection is affected by interference from other users, the cell can be divided into sectors, where each sector is separated from other sectors by a unique or phase-shifted code.
This is illustrated in FIG. 1A which shows a number of BSs 120, each serving at least one cell 100, each cell containing at least one sector 110. For simplicity of illustration, only one cell is shown divided into sectors in FIG. 1A. Each sector antenna is exposed mainly to interference in the direction of the current sector. Thus, using a directional antenna largely avoids interference from other users, increasing the capacity of the cellular system. Due to the division of cells into sectors, the remote station 130 not only needs to identify the BS serving a cell but also the sector of the cell in which the remote station can transmit signals to and receive signals from the BS.
The signal strength of the signal received by the remote station 130 from a particular BS 120 may decrease for a number of reasons. For example, if the remote station is mobile, the signal strength may decrease as the remote station moves away from the BS. If the remote station is fixed, the signal strength may decrease if there is a problem at the BS and/or if new users interfere. In such cases, when the signal quality degrades, the remote station may need to be redirected to another BS. This redirection of the connection between the remote station and the BS is referred to as handoff. The performance of the connection between the remote station and the BS can be improved with respect to diversity gain, when the remote station 130 is connected to more than one BS at the same time. This can be referred to as macrodiversity. It may also be necessary to redirect a connection between a remote station and a particular BS to another sector served by the same BS. This is sometimes referred to as softer handoff. For handoff to be effective, the remote station must be able to identify a new sector and BS to which its connection will be redirected and to synchronize with the BS serving that sector. Ideally, handoff should be unnoticed (seamless) to the user.
FIG. 1B illustrates a basic frame structure of a downlink informational datastream for a communication system, such as a system according to the developing IMT2000 standard. A frame may include sixteen slots and a total of 40960 complex chips, i.e., 40960 Q-ary symbols of the high rate coded signal. Each slot may include a number of pilot symbols and a varying number of information symbols determined by the current spreading factor (sf). The pilot symbols can be used by a receiver, for example, to perform channel estimation. Each slot may include 2560 chips.
Further details regarding the frame and channel structure for the IMT2000 standard are described in IMT-2000 Study Committee Air-interface WG, SWG2, xe2x80x9cVolume 3 Specifications of Air-interface for the 3GMobile Systemxe2x80x9d, Ver. 0-3. Dec. 1, 1997 and in xe2x80x9cUTRA Physical Layer Description FDD parts (vo. 1, Apr. 24, 1998)xe2x80x9d, ETSI SMG2 UMTS Physical Layer Expert Group, Tdoc SMG2 UMTS-L1 56/98, Meeting 2, Paris, France, Apr. 28, 1998, which are expressly herein incorporated by reference.
Referring again to FIG. 1A, a BS 120 can transmit signals to one or more remote stations 130 as a single (composite) signal. The signal directed to a remote station 130 is typically spread with a Short Code that is orthogonal or mostly orthogonal to a Short Code that is used to spread the signal directed to another remote station 130. These signals are then scrambled with a second code that is sometimes referred to as a Long Code, associated with a particular BS 120. The sum of a plurality of spread and scrambled signals is then transmitted by the BS 120.
When a remote station 130 receives the composite signal, the remote station multiplies the spread signal with the Long Code and the Short Code to recreate the signal directed to that remote station, and the signal directed to the other remote station is suppressed as interference noise. Similarly, the other remote station multiplies the spread signal with the Long Code and the Short Code assigned to it to recreate the signal directed to it, and the signal directed to the other remote station is suppressed as interference noise. The receivers associated with the remote stations 130 must have acquired various levels of synchronization to the received signal, in addition to learning or knowing the applicable Long and Short Codes, in order to implement despreading, demodulation, and decoding of the information residing in that signal.
For downlink synchronization to the remote station 130, each BS 120 periodically transmits primary and secondary synchronization codes, for example, a Primary Synchronization Code (PSC) and a Secondary Synchronization Code (SSC) (or a Primary Synchronization Channel (SCH) and a Secondary SCH according to a developing ETSI standard) in downlink physical channels. For ease of explanation, reference will be made to the PSC and the SSC. For base station identification, the downlink information includes the Long Code. The Long Code is cyclically repeated every frame, i.e., the Long Code resets for each frame.
The PSC and the SSC are transmitted in primary and secondary control channels, e.g., a Perch 1 channel and a Perch 2 channel (or a Primary Common Control Physical Channel (CCPCH) and a Secondary CCPCH according to a developing ETSI standard), respectively. For ease of explanation, reference will be made to Perch 1 and Perch 2 channels. A Perch channel is a one-way physical channel from the BS to the remote station that the remote station uses, for example, for received signal strength measurement and cell selection. FIG. 2A illustrates the timeslot format of Perch 1 and Perch 2 channels. The duration of a timeslot may be, for example, 0.625 msec.
In the Perch 1 channel, which is a broadcast channel, each slot includes four pilot symbols and five information symbols. These nine symbols are scrambled by the Long Code. The Long Code may be an extended Gold Code. The remaining symbol is a Long Code Mask Symbol (LCMS). This symbol is Long Code Masked, i.e., it is not scrambled by the Long Code. Instead, the LCMS in the Perch 1 channel includes the PSC. The LCMS in the Perch 1 channel may comprise 256 chips arranged in a pattern such as an extended Gold sequence.
In the Perch 2 channel, which is also a broadcast channel, there are nine xe2x80x9cidlexe2x80x9d symbols, i.e., nine symbols in which no information is transmitted, and one LCMS. None of the symbols in the Perch 2 channel are scrambled by the Long Code. The LCMS in the Perch 2 channel includes the SSC.
A more detailed description of the PSC and the SSC is given in a copending and commonly assigned U.S. patent application Ser. No. 08/921,135, filed Aug. 29, 1997, in the names of Karim Jamal et al., which is expressly herein incorporated by reference.
A logical Broadcast Control Channel (BCCH) is mapped, for example, to the information symbols in the Perch 1 channel. The BCCH is broadcast from all sectors to the remote stations to deliver cell-specific information, e.g., cell identification and sector identification, and system-related information, e.g., transmit power, uplink interference power, etc. Sector identification has conventionally been performed by decoding and deinterleaving the BCCH symbols. Since the BCCH symbols also contain information that is not necessary for sector identification, e.g., power information, this conventional method for sector identification consumes an unnecessarily large amount of processing time and resources.
In order to establish or redirect a connection with a base station, the remote station needs to identify the sector to which the connection is to be redirected or in which the connection is to be established. The remote station 130 also needs to know both the slot and frame boundaries of the downlink informational datastream, for synchronization with the timing reference of the BS. For effective and seamless handoff between sectors, synchronization and identification should be performed as fast as possible, with a minimum amount of effort.
It is therefore an object of the present invention to identify a sector to which a connection with a remote station is to be redirected or in which a connection with a remote station is to be established and to synchronize the remote station with the timing reference of the BS serving that sector as fast as possible and with a minimum effort for the remote station.
This and other objects are met by a method, apparatus, and system for identifying a sector to which a connection with a remote station is to be redirected or in which a connection with a remote station is to be established and synchronizing the remote station to the BS serving the sector.
According to exemplary embodiments, the base station transmits information in primary and secondary control channels to the remote station. The remote station performs synchronization using information in the primary and secondary control channels. A group of identification codes corresponding to the sector is determined using information in the secondary control channel. If this does not result in identification of the sector, an identification code corresponding to the sector is determined from information in the primary control channel. If this does not result in identification of the sector, e.g., because different sectors share the same identification code, the sector is identified based on information multiplied with symbols, e.g., pilot symbols, in the primary control channel, without having to decode the BCCH.
According to exemplary embodiments, the secondary control channel carries a sequence of symbols repeated in every frame, e.g., a sequence of symbols from a binary, Q-ary, or M-ary alphabet. According to a first embodiment, the sequence of symbols is from an m-sequence. According to a second embodiment, the sequence of symbols is from a group of (16,2) Reed Solomon (RS) code words. According to a third embodiment, the sequence of symbols is from a group of (16,3) RS code words. According to a fourth embodiment, the sequence of symbols is from a group of (16,4) RS code words. The sequence of symbols can also be taken from other types of code word groups, e.g., Hamming code word groups.